Some hydraulic pistons used in a cylinder block of a hydrostatic power unit are of solid steel construction. While they are durable, reliable, and inexpensive to make, their relative greater weight imposes limitations on their speed of operation. They also develop more operational frictional forces.
Alternatively such pistons are constructed with a hollow cavity therein to reduce the weight of the solid steel pistons. However, the hollow cavities become filled with oil which is compressible. The resulting compression and de-compression of the oil within the piston cavity during pressure cycles results in inefficient energy consumption and has an adverse affect on the moments of inertia imposed on the swashplate with which the pistons interact.
The shortcomings of the hollow pistons are overcome in part by pistons which fill the hollow cavity with a plastic material which is lightweight and which is not as compressible as oil. However, it is difficult to get reliable material that can endure the harsh environment of the pistons whereupon deterioration of the plastic material is common. Aluminum slugs in the cavity instead of the plastic material are more durable, but they are difficult to retain within the cavity.
Some pistons provide a hollow cavity with a cap on one end to close the cavity. Oil does not dwell within the cavity because a hollow tube extends through the piston and through the cavity to prevent the cavity from becoming filled with oil. The tubes within the cavity are often reinforced by a washer or the like which spans the distance from the outer diameter of the tube to the inner diameter of the cavity. While pistons of this design solve at least some of the problems outlined heretofore, they are very expensive to make.
Therefore, a principal object of this invention is to provide a closed cavity hydraulic piston and a method of making the piston which is economical of manufacture and which overcomes the above-described shortcomings of existing pistons.
More specifically, it is an object of this invention to provide a closed cavity hydraulic piston which has a hollow piston body with a piston cap thereon which are both formed by a metal injection molding process.
A further object of this invention is to provide a closed cavity hydraulic piston and method of manufacture which provides for adhering the cap to the piston body while simultaneously hardening the piston body and leaving the cap malleable for crimping or swaging.
A further object of this invention is to provide a closed cavity hydraulic piston wherein oil conduits are formed in the piston body and the cap separate from the cavity wherein the cap is adhered to the piston body for closing the cavity and wherein the cap is self-aligned with the piston body by reason of complementary engagement surfaces on the cap and the piston body.
A still further object of this invention is to provide a closed cavity hydraulic piston and method of making the same which will essentially eliminate finishing operations through the fabrication of the body and the cap by a metal injection molding process.
A still further object of the invention is to provide a hydraulic piston which is quiet in operation, capable of being operated at high speeds, energy efficient in operation, and inexpensive of manufacture.
These and other objects will be apparent to those skilled in the art.
A hydraulic piston for a cylinder block of a hydrostatic power unit has an elongated cylindrical body with first and second ends. An elongated cavity is formed in the body. A cap is adhered to the body to close the cavity. The cap has an external shape to facilitate the operative engagement thereof with a slipper which is in subsequent engagement with a swashplate mounted adjacent the cylinder block
The piston has an elongated first conduit formed in the body which is separate from the cavity and extends between the opposite ends thereof. The conduit terminates in registering communication with a second conduit in the cap thereof to permit transmission of oil through the conduit without invading the volume of the cavity.
The cap preferably has a tapered surface on the inner end thereof engaging a compatible shaped surface on an adjacent end of the body to center and align the cap with the body. The tapered surfaces of the cap and body terminate in parallel wall surfaces to create a continuous arcuate interface therebetween which is filled with an adhesive material such as a brazing medium.
The body member and the cap are of metallic construction with the material of the cap member being more malleable than the material of the body member so as to permit subsequent crimping of the cap as may be required.
The body and the cap are preferably formed by a metal injecting molding process utilizing a molding material comprised at the time of molding of a mixture of metallic grains and a binder material. The body member and the cap are subjected to a de-binding process whereupon the binder material is removed from the molding material and the components are then sintered, whereupon controlled shrinking occurs.
The cap is adapted to terminate either in a spherical ball for attachment to a slipper element, or is comprised of a spherical socket adapted to receive a ball element attached to a slipper element.